I will have to tell you what the driveline shop will tell you. Once you have the truck lifted, make sure it weighs approximately what it will when being driven regularly. Then, measure from the center of the pinion yoke to the center of the rear output shaft yoke (to the center of where the U-joint trunion would be). Do the same for the front, except you will measure to where the front CV joint meets up against the 'case. '79 GMC shortbed 454 th400 14 bolt FF NP205 Dana 60
Oh yeah, I dopn't know if oyu have the 2 piece driveshaft or not. If so, get rid of the carrier bearing and have a one piece shaft built. '79 GMC shortbed 454 th400 14 bolt FF NP205 Dana 60